This invention relates to blood glucose testing in critically ill patients. The need for a convenient and easily applied method of glucose monitoring in the Intensive Care Unit became evident after the landmark study of Van den Berghe and colleagues published in the Nov. 8, 2001, issue of The New England Journal of Medicine. 
This paper demonstrated an overall reduction in ICU patient mortality of 34% when blood glucose was kept in the 80 to 110 mg per deciliter range. Samples were taken from an arterial line at 1 to 4-hour intervals and sent to the hospital lab for analysis. In the intensive therapy group, an insulin infusion was started if blood glucose exceeded 110 mg per deciliter and was adjusted to maintain normal blood glucose levels. A virtual flood of articles have since appeared and confirm improved outcomes in the treatment of various critical conditions including infection, stroke, in patients undergoing coronary bypass surgery, and in the treatment of myocardial infarction in both diabetic and non-diabetic patients. One study showed greatly improved outcomes when diabetics were monitored and treated intensively with insulin in the hospital for three days prior to undergoing coronary bypass surgery.
Intensive treatment with insulin requires knowledge of patient blood sugar levels which presently involves obtaining either an arterial or a venous blood sample or pricking the patient's finger to obtain a capillary blood sample. Blood samples are placed on a strip and read using a home-type glucose meter. All of these methods require considerable nurse or technician time. In the U.S. at present only 20 to 30% of patients in the ICU have arterial lines. Many patients, especially non-diabetics, find repeated finger sticks objectionable. Furthermore, intermittent blood samples may not be done often enough to give an accurate picture of blood sugar levels.
Optimally, patients in critical care situations should have blood glucose levels monitored several times each hour so that insulin can be given appropriately to keep blood sugar readings in the narrow range of 80-110 mg/deciliter. Presently there is no system available that accurately and conveniently measures blood sugar without taking frequent blood samples from a patient. The present invention is able to do so by withdrawing blood from a central venous line, an arterial line or a catheter in a peripheral vein. The catheterized blood vessel in any of these locations is normally used for infusion of fluids with electrolytes or various medications. The present invention also allows these lines to monitor a patient's blood sugar as often as every three minutes so that a care giver can adjust insulin dosage as required.
Attempts have been made in the past to automatically monitor blood analytes from a patient's IV line. Generally these systems have used reversal of the direction of flow in an infusion line so that blood could be pulled out of the patient's circulation at intervals, analyzed, and then re-infused back into the patient. Examples of such device are described in U.S. Pat. No. 3,910,256 to Clark, U.S. Pat. No. 4,573,968 to Parker and U.S. Pat. Nos. 5,165,406, 5,758,643 and 5,947,911 to Wong and associates.
The devices described in the above patents measure blood analytes with sensors inside the main infusion line and no attempt has been made to isolate or compartmentalize blood samples during testing. This technique, while successful for monitoring blood gases, has not yet met with success in testing for blood glucose. The present invention, by novel means, allows practical and accurate monitoring of blood sugar from a reversible infusion line.
State of the art blood glucose measuring systems generally employ the enzymes glucose oxidase or glucose dehydrogenase. In the chemical reaction which occurs using either of these two enzymes, glucose is consumed and the electrons generated are drawn off and measured. Plotting current flow against time produces a curve which is distinctive for each glucose value, i.e., any given concentration level of blood glucose.
Accuracy depends crucially on maintaining a constant diffusion gradient over the glucose sensor. In the ideal case the concentration of glucose varies in a regular and linear way from the body's reservoir of blood glucose to the area just over the sensor where concentration drops to a minimum (close to zero) because it is being consumed in the chemical reaction. The major cause of inaccuracy in electrochemical glucose measurement is disturbance of the diffusion gradient. The problem is relatively small in single use disposable strips because a portion of a blood drop is allowed to rest undisturbed in a small channel during the test, having been drawn up into the channel by capillary attraction. In a flow-through system the problem of leaving the diffusion gradient undisturbed is a major one because of the much larger volume of fluid involved, which is essentially all the fluid in the infusion line from the patient to the bedside monitor.
A disturbance anywhere in this large reservoir of fluid is immediately transmitted to the test area and causes disturbance of the diffusion gradient over the sensing electrode. Invariably, the effect is to bring more glucose to the region of the electrode, causing an increase in the glucose signal and an overestimation of blood glucose.
A number of causes can contribute to disruption of the diffusion gradient in a flow through system. Any fluid movement, even a flow rate as low as 150 micro liters/minute, can cause a mixing effect which disturbs the diffusion gradient and gives an overestimation of blood glucose. Eddies in the fluid line can occur because of the intermittent “push” of a peristaltic pump. Such eddies can last several seconds after pumping has stopped.
Adjustment of blood temperature to ambient temperature can cause micro fluidic deviations which will disturb the diffusion gradient. Heat transfer issues are minimized by keeping the amount of fluid small in comparison to the total mass of the device. The device can be subjected to temperature controls but under practical conditions small volumes are required to prevent temperature differences from causing significant measurement errors. The diffusion gradient over the sensor can also be disturbed in an open-flow system by movement of the arm or chest, or changes in the relative position of the test chamber and the heart. Additionally, impacts to the tubing or glucose test area can cause measurement inaccuracies if the fluid in the test area is not protected in a small, well defined space.
For complete isolation, small valves, which may be inflatable balloons, can be located adjacent the test chamber. Besides protecting the diffusion gradient, isolation of the test sample prevents movement of glucose molecules into or out of the test area and allows consistent measurement of the current produced during the reaction at any given glucose level.
In the present invention, the test chamber for testing blood glucose can either be part of the main infusion line inside the testing unit, or it can be located in a side channel in the testing unit in continuity with the main channel. In the latter version, a valve, which may be an inflatable balloon, directs blood or fluid into the side channel at the time of testing. For reasons to be enumerated, the side channel version is considered the preferred embodiment of the invention.
The system to be described uses a bedside monitor with a digital readout and alarms and contains two peristaltic pumps, one of which normally infuses fluid through the blood vessel catheter. During glucose testing the pump assists in moving blood samples into and out of the test chamber. The second peristaltic pump automatically calibrates the sensor at intervals with a premixed calibration fluid.
The disposable, single patient use testing unit is approximately 2¼″×1½″×¾″ and is attached to the patient's chest for use with a central line or to an extremity for use with a catheter in a peripheral vessel. The distal end of the testing unit has an integral Luer fitting which connects to the patient's blood vessel catheter. The proximal end of the testing unit has the exit sites for the fluid, air, and electric lines that connect to a bedside monitor located a few meters from the patient.
The disposable testing unit portion of the invention is made of a semi-transparent plastic. It contains a channel for infusion fluid in continuity with (i.e. in fluid communication with) the infusion line and with the blood vessel catheter. A glucose sensing electrode is located in the test chamber area of the testing unit. Chambers for inflatable balloons, which serve as valves, are molded into the plastic parts along with grooves for the air lines and the electric cable. After insertion of the various components during manufacturing, the top and bottom halves are welded together to form a leak proof disposable testing unit. While inflatable balloons are considered the preferred embodiment for reasons of economy and ease of insertion, mechanical valves can also be used for this application.
The sensor of the present invention can be automatically calibrated at selected intervals using a small bag of premixed calibration fluid which is supplied with each disposable testing unit. The calibration fluid is carried inside the monitor and made to flow through the test chamber at intervals by a second peristaltic pump. After calibration is complete the test chamber is cleared by a brief flow of infusion fluid through the sampling area.
The area of the glucose sensor in the present invention is approximately 25 square millimeters, considerably larger than that of a typical disposable strip. The height of the test chamber's is from 0.3 to 0.5 mm, giving a fluid volume in the chamber of 8 to 12 micro liters. The large electrode described herein is thought advantageous in this specific application because of the large amount of current produced during a test. Current from a 25 sq. millimeter sensor will be measured in micro amps rather than the nano amps of some single use strips. A large current flow is advantageous for optimum resolution of the signal.
In the present invention, the openings into the test chamber are at least 300 microns in height, which duplicate the height of the test chamber. Blood is quite viscous and this minimum height is necessary to allow adequate blood flow in and out of the test chamber. Positive pressure is still required to bring blood or fluid into the test chamber and for the same reason pressure is needed to clear the site once the test has been performed. The present invention is capable of exerting adequate pressure to draw in the sample or flush it from the test chamber.
In the descriptions and drawings to follow, two embodiments of the invention are shown. The first embodiment has its test chamber located directly in the main infusion line. The second embodiment has the test chamber located in a side channel, in continuity with the main channel. Several advantages accrue to the side channel version. Firstly, an unrestricted main channel (used together with a restricted side channel) allows for the rapid flow of fluid through the device most of the time. Secondly, a constant flow of fluid through the test chamber (if no side channel is used) could disperse the reagents necessary for repeated testing of blood glucose. This problem is avoided, in the preferred embodiment, since infusion fluid usually flows through the main channel. Thirdly, in the non-side channel version, a large volume of blood must be forced through the test chamber so that an undiluted sample is adjacent the sensor. Damage to red cells can occur when forcing a large quantity of blood through a very small opening. By contrast, in the side channel version blood can be easily withdrawn from the patient through the unobstructed main channel until a pure sample is opposite the test site. At that time only a very small quantity of blood need be directed into the side channel for testing.
It should be emphasized that a controllable valve in the main channel is an essential aspect of the side channel version of the invention. Without controlled blockage of the main channel, blood and fluids would always take the path of least resistance through the main channel and never enter the side channel.
In the following description of the present invention, the one or more valves inside the device are comprised of inflatable balloons. It is to be understood that mechanical valves of various types could also be used in this application. Inflatable balloons are considered the preferred embodiment for reasons of economy and for ease of insertion during assembly.
In the examples to follow, traditional monitoring in an open-flow system is compared to testing with the preferred embodiment of the present invention. Monitoring blood sugar in a conventional flow-through system is possible if there is no disturbance of the diffusion gradient over the sensing electrode. In reality, such disturbances occur constantly for the several reasons mentioned. In a flow-through system in which blood or fluid is propelled by a peristaltic pump, the cog-wheel effect is very evident and causes confusing variations to the glucose diffusion gradient. Additionally, a peristaltic flow-through system is subject to all the additional artifacts contributed by impacts to the infusion bag, the tubing and the sensor itself. Clearly a sensor which isolates the test fluid and keeps it completely at rest is best able to maintain a stable diffusion gradient and give the most accurate estimation of blood glucose.
No references in the prior art have been found to methods of isolating small extra corporal samples for testing. Therasense U.S. Pat. No. 6,120,676 states that a sensor for blood glucose can be used in a flowing sample stream which is made to flow through a sampling chamber. No method is described of sample isolation although the authors state that the sample can be made to flow at a slow rate. In claim 32 of the same patent the authors mention holding the sample stationary in a sampling chamber but the latter is on a disposable strip and not in a reversible infusion line where the sensor must test repeatedly over a period of days.
An advantage of the present invention is to put blood in contact with the sensor only briefly during each test. Following the test, the test chamber in the testing unit is flushed with clear fluid which reduces protein and fibrin deposition on the sensing membrane. For example, if a glucose test takes 20 seconds and readings are done every five minutes, then blood will be in contact with the sensor only about 3% of the total elapsed time.
A primary object of the invention is to provide a system for repeated monitoring of blood glucose from an ordinary infusion line with an accuracy approaching that of a clinical lab.
A further object of the invention is to attain such accuracy by isolation of the test sample during a test.
A further object of the invention is a system usable with equal ease in either a peripheral vein or a central venous line.
A further object of the invention is to provide a small disposable testing unit which is attachable to either the chest for proximity to a central line or to an extremity for use with a peripheral vein.
A further object of the invention is to provide a system which is automatically and periodically self-calibrating.
Other objects and advantages of the invention will become apparent from the following description and drawings.